U.S. application Ser. No. 323,920, filed concurrently herewith in the names of Ronald L. Andrus, Kenneth Chyung, and Richard F. Reade under the same title as the instant application, provides a discussion of the development of electronic circuit boards. Thus, as is explained therein, circuit boards are essentially a substrate coated with a dielectric material upon the surface of which is applied the desired conductive metal circuitry. The substrates or boards, as they have been familiarly termed, have customarily been fabricated from three general categories of materials, viz., organic plastics, ceramic wafers, and porcelain-coated steel. That application describes the deficiencies and shortcomings exhibited by each of those materials.
That application also reviews the disclosure of U.S. Pat. No. 4,256,796 which is drawn to the fabrication of porcelain-coated, metal circuit boards wherein the "porcelain" is a devitrified glass having a composition which is substantially free from alkali metal oxide and consists essentially, expressed in terms of mole percent on the oxide basis, of
______________________________________ BaO 6-25 MgO + optionally CaO and/or ZnO 30-60 B.sub.2 O.sub.3 13-35 SiO.sub.2 10-25 ______________________________________
Whereas a direct conversion cannot be made of ranges expressed in mole percent to ranges stated in weight percent, the following table sets forth an approximation of the disclosed compositions (also utilizing the exemplary compositions tabulated in the patent) in terms of weight percent.
______________________________________ BaO 16-50 MgO 16-42 CaO 0-11 ZnO 0-11 CaO + ZnO 0-11 B.sub.2 O.sub.3 12-34 SiO.sub.2 10-23 ZrO.sub.2 0-5 Al.sub.2 O.sub.3 0-5 SnO.sub.2 0-5 ZrO.sub.2 + Al.sub.2 O.sub.3 + SnO.sub.2 0-5 ______________________________________
The patent describes melting batches for the above compositions, forming glass frits from the melts, applying a coating of the frits onto the surface of a metal substrate (most preferably low carbon steel), and then firing the coated substrate at a temperature of at least 750.degree. C., more desirably 800.degree.-850.degree. C., for a period of time sufficient to cause the frit to essentially simultaneously sinter together to an integral mass and crystallize in situ, this time commonly ranging from 5-30 minutes. The resultant coating is highly crystalline, i.e., about 50-90% by volume crystalline, with residual glass comprising the remainder thereof. The primary crystal phase is BaO.2MO.2SiO.sub.2, wherein MO consists of MgO with, optionally, CaO and/or ZnO. 2MO.B.sub.2 O.sub.3 constitutes the secondary crystal phase wherein MO again consists of MgO with, optionally, CaO and/or ZnO. The final coatings demonstrate high coefficients of thermal expansion, typically 110.times.10.sup.-7 /.degree.C. and above, over the interval of 0.degree. C. to the deformation temperature of the material (&gt;700.degree. C.). Such coefficients of thermal expansion are useful in matching the high coefficients of thermal expansion of the metal substrates employed, e.g., the preferred low carbon steels.
Application Ser. No. 323,920, supra, discloses glass-ceramic coatings capable of being applied to metal substrates which are more refractory than the coatings of Pat. No. 4,256,796 and demonstrate coefficients of thermal expansion (25.degree.-600.degree. C.) ranging between about 80-125.times.10.sup.-7 /.degree.C. Those coatings are essentially free from alkali metal oxides and consist essentially, expressed in terms of weight percent on the oxide basis, of
______________________________________ MgO 5-35 CaO 0-35 ZnO 0-15 CaO + ZnO 10-35 Al.sub.2 O.sub.3 0-10 B.sub.2 O.sub.3 0-25 SiO.sub.2 25-50 P.sub.2 O.sub.5 0-10 B.sub.2 O.sub.3 + P.sub.2 O.sub.5 4-25 ______________________________________
The coatings are prepared in a manner similar to that described in U.S. Pat. No. 4,256,796 but the combined sintering and crystallizing step is carried out at temperatures of at least 850.degree. C. The crystal phases generated are dependent upon the presence of CaO and/or ZnO. The occurrence of such phases as diopside (CaO.MgO.2SiO.sub.2), magnesium borate (2MgO.B.sub.2 O.sub.3), willemite (2ZnO.SiO.sub.2), akermanite (2CaO.MgO.2SiO.sub.2) and enstatite (MgO.SiO.sub.2) is noted. The coatings can be fired to temperatures up to and in excess of 1000.degree. C. with no deformation or other deleterious effect being observed.